The influence of arterial oxygenation on cerebral venous oxygen saturation during hyperventilation

Abstract

Cerebral venous oxygen desaturation may occur when hyperventilation is employed during neurosurgical procedures. In this study, we examined the effect of arterial hyperoxia (PaO2 > 200 mmHg) on jugular bulb venous oxygen tension (PjvO2), saturation (SjvO2) and content (CjvO2) in 12 patients undergoing anaesthesia for neurosurgical procedures. Under stable anaesthetic conditions, the inspired oxygen fraction (FlO2) was varied to give four different levels of arterial oxygen tension (PaO2 100-200, 201-300, 301-400, and > 400 mmHg), at two levels of controlled hyperventilation (PaCO2 25 and 30 mmHg). In five patients, a transcranial Doppler probe was used to insonate the middle cerebral artery throughout the study period. Regression lines were constructed for each patient for the PjvO2, SjvO2 and the corresponding PaO2 for both levels of PaCO2 (all PjvO2-PaO2 and SjvO2-PaO2 regression lines r2 > 0.85, P < 0.0001). From these lines we calculated the PjvO2, SjvO2 and CjvO2 at PaO2 of 100, 250 and 400 mmHg, at each level of PaCO2 for each patient. At PaCO2 of 25 mmHg, hyperoxaemia increased PjvO2 (from 27.6 ±1.1 mmHg at PaO2 of 100 mmHg to 30.6 ± 1.4 and 33.6 ± 1.8 mmHg at PaO2 of 250 and 400 mmHg respectively) and SjvO2 (from 54 ± 3% at PaO2 of 100 mmHg to 60 ± 3 and 65 ± 3% at PaO2 of 250 and 400 mmHg respectively, P < 0.05). Hyperoxaemia had a similar effect on SjvO2 and PjvO2 at a PaCO2 of 30 mmHg. For a given PaO2, the PjvO2, SjvO2 and CjvO2 were lower at PaCO2 of 25 mmHg than at a PaCO2 of 30 mmHg (P < 0.01). The predicted CjvO2 based on the increased PaO2 and an unchanged cerebral metabolic rate for oxygen was also calculated and was no different from the measured CjvO2 with hyperoxia. Middle cerebral artery flow velocity did not change with hyperoxia, but decreased with hypocapnia (48 ± 7 to 35 ±4 cm· sec-1, P< 0.01). We conclude that hyperoxia during acute hyperventilation in the anaesthetized patient improves oxygen delivery to the cerebral circulation, as measured by a higher cerebral venous oxygen content and saturation. An increased PaO2 should be considered for those patients in whom aggressive hyperventilation is contemplated. © 1994 Canadian Anesthesiologists.